Method of producing cubic boron nitride sintered material, cubic boron nitride sintered material, and cutting tool including cubic boron nitride sintered material

ABSTRACT

A method of producing a cubic boron nitride sintered material includes: forming an organic cubic boron nitride powder by attaching an organic substance onto a cubic boron nitride source material powder; preparing a powder mixture including more than or equal to 85 volume % and less than 100 volume % of the organic cubic boron nitride powder and a remainder of a binder source material powder by mixing the organic cubic boron nitride powder and the binder source material powder, the binder source material powder including WC, Co and Al; and obtaining the cubic boron nitride sintered material by sintering the powder mixture.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on PCT filing PCT/JP2019/036569, filedSep. 18, 2019, which claims priority to JP 2018-174695, filed Sep. 19,2018, the entire contents of each are incorporated herein by reference.This disclosure is also related to co-pending U.S. application Ser. No.17/262,215, which is entitled: CUBIC BORON NITRIDE SINTERED BODY,CUTTING TOOL CONTAINING THIS, AND PRODUCTION METHOD OF CUBIC BORONNITRIDE SINTERED BODY, filed concurrently with the present application,which is also incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a method of producing a cubic boronnitride sintered material, the cubic boron nitride sintered material,and a cutting tool including the cubic boron nitride sintered material.

BACKGROUND ART

A cubic boron nitride sintered material (hereinafter, also referred toas “cBN sintered material”) is a high-hardness material used for cuttingtools and the like. The cubic boron nitride sintered material isnormally constituted of cubic boron nitride grains (hereinafter, alsoreferred to as “cBN grains”) and a binder. Depending on a content ratioof the cubic boron nitride grains, characteristics of the cBN sinteredmaterial tend to differ.

Hence, in the field of cutting, different types of cubic boron nitridesintered materials are applied to cutting tools in accordance with thematerial of a workpiece, required precision in processing, or the like.For example, a cubic boron nitride sintered material (hereinafter, alsoreferred to as a “high-cBN sintered material”) having a high contentratio of cubic boron nitride (hereinafter, also referred to as “cBN”)can be suitably used for cutting of a sintered alloy or the like.

However, the high-cBN sintered material tends to be likely to beunexpectedly chipped. Such sporadic chipping is considered to be causeddue to the following reason: binding strength between the cubic boronnitride grains is weak to result in falling of the cubic boron nitridegrains. For example, WO 2005/066381 (PTL 1) discloses a technique ofsuppressing occurrence of sporadic chipping in a high-cBN sinteredmaterial by appropriately selecting a binder.

CITATION LIST Patent Literature

-   PTL 1: WO 2005/066381

SUMMARY OF INVENTION

A method of producing a cubic boron nitride sintered material accordingto one embodiment of the present disclosure includes: forming an organiccubic boron nitride powder by attaching an organic substance onto acubic boron nitride source material powder; preparing a powder mixtureincluding more than or equal to 85 volume % and less than 100 volume %of the organic cubic boron nitride powder and a remainder of a bindersource material powder by mixing the organic cubic boron nitride powderand the binder source material powder, the binder source material powderincluding WC, Co and Al; and obtaining the cubic boron nitride sinteredmaterial by sintering the powder mixture.

A cubic boron nitride sintered material according to one embodiment ofthe present disclosure includes: more than or equal to 85 volume % andless than 100 volume % of cubic boron nitride grains; and a remainder ofa binder, wherein the binder includes WC, Co and an Al compound, andwhen a TEM-EDX is used to analyze an interface region including aninterface at which the cubic boron nitride grains are adjacent to eachother, carbon exists on the interface, and a width D of a region inwhich the carbon exists is more than or equal to 0.1 nm and less than orequal to 10 nm.

A cubic boron nitride sintered material according to another embodimentof the present disclosure includes: more than or equal to 85 volume %and less than 100 volume % of cubic boron nitride grains; and aremainder of a binder, wherein the binder includes WC, Co and an Alcompound, when a TEM-EDX is used to analyze an interface regionincluding an interface at which the cubic boron nitride grains areadjacent to each other, carbon exists on the interface, and a width D ofa region in which the carbon exists is more than or equal to 0.1 nm andless than or equal to 5 nm, and a maximum value M of a content of thecarbon in the region in which the carbon exists is more than or equal to0.1 atom % and less than or equal to 5.0 atom %.

A cutting tool according to one embodiment of the present disclosure isa cutting tool including the above-described cubic boron nitridesintered material.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an exemplary second image.

FIG. 2 shows an exemplary element distribution based on a result ofelement mapping analysis with an image indicating a distribution stateof boron.

FIG. 3 shows an exemplary element distribution based on a result ofelement mapping analysis with an image indicating a distribution stateof nitrogen.

FIG. 4 shows an exemplary element distribution based on a result ofelement mapping analysis with an image indicating a distribution stateof carbon.

FIG. 5 is an exemplary graph showing a result of line analysis.

DETAILED DESCRIPTION Problem to be Solved by the Present Disclosure

In recent years, due to rapid improvement in functions of mechanicalparts, it becomes more difficult to cut workpieces to serve as themechanical parts. This leads to a short life of a cutting tool, thusapparently resulting in increased cost, disadvantageously. Therefore,further improvement of a high-cBN sintered material has been desired. Inview of this, it is an object of the present disclosure to provide amethod of producing a cubic boron nitride sintered material having along life, the cubic boron nitride sintered material, and a cutting toolincluding the cubic boron nitride sintered material.

Advantageous Effect of the Present Disclosure

According to the cubic boron nitride sintered material obtained asdescribed above, a long life can be attained, with the result that thelife of the cutting tool including the cubic boron nitride sinteredmaterial can also be long.

Description of Embodiments

In order to complete a cubic boron nitride sintered material having alonger life, the present inventors first have decided to use a bindersource material powder including WC (tungsten carbide), Co (cobalt), andAl (aluminum) as a source material of the binder in the high-cubic boronnitride sintered material. This is because the present inventors haveobtained the following knowledge in previous research: when such abinder source material powder is used, the binder has a particularlyhigh binding strength with respect to cubic boron nitride grains, withthe result that an excellent cubic boron nitride sintered material canbe obtained.

However, in the high-cBN sintered material, the amount of the binder issignificantly smaller than the amount of the cubic boron nitride grains.Hence, the binder tends to be less likely to be distributed widelybetween the cubic boron nitride grains. Therefore, the present inventorshave considered that any breakthrough for attaining a long life of thehigh-cBN sintered material cannot be achieved only by optimizing thebinder.

Therefore, from a greatly different point of view, the present inventorshave sought for a technique of increasing binding strength between thecubic boron nitride grains, unlike the conventional technique ofincreasing binding strength between the binder and each of the cubicboron nitride grains. As a result of various studies, it has been foundthat a small amount of carbon between the cubic boron nitride grainsleads to increased binding strength between the cubic boron nitridegrains. However, when the amount of carbon between the cubic boronnitride grains is large, the characteristics of the cubic boron nitridesintered material are changed.

In view of the above knowledge, in order to significantly increase thebinding strength between the cubic boron nitride grains by disposingcarbon between the cubic boron nitride grains, the present inventorshave considered it necessary to uniformly dispose carbon between thecubic boron nitride grains without the carbon existing in an imbalancedmanner. In order to find a technique to attain this, the presentinventors have conducted diligent study. As a result of the diligentstudy, the present inventors have found a technique of uniformlyattaching an organic substance onto surfaces of particles of a cubicboron nitride source material powder, and also have found that carboncan be uniformly disposed between cubic boron nitride grains byproducing a cubic boron nitride sintered material using the cubic boronnitride source material powder.

The present disclosure has been completed in the manner described above.Hereinafter, embodiments of the present disclosure will be listed anddescribed.

[1] A method of producing a cubic boron nitride sintered materialaccording to one embodiment of the present disclosure includes: formingan organic cubic boron nitride powder by attaching an organic substanceonto a cubic boron nitride source material powder (forming step);preparing a powder mixture including more than or equal to 85 volume %and less than 100 volume % of the organic cubic boron nitride powder anda remainder of a binder source material powder by mixing the organiccubic boron nitride powder and the binder source material powder, thebinder source material powder including WC, Co and Al (preparing step);and obtaining the cubic boron nitride sintered material by sintering thepowder mixture (sintering step).

According to the production method, a cubic boron nitride sinteredmaterial having a long life can be produced. This is presumably due tothe following reason. First, the organic cubic boron nitride powder inwhich the organic substance is uniformly attached on the surfaces of theparticles of the cubic boron nitride source material powder is formed bythe forming step. Then, the powder mixture including the organic cubicboron nitride powder is prepared in the preparing step. In thesubsequent sintering step, the cubic boron nitride sintered material isproduced. In the sintering step, carbon on the surfaces of the particlesof the organic cubic boron nitride powder exhibits a catalyst function.

Here, the catalyst function of the carbon refers to diffusion orprecipitation of B (boron) and/or N (nitrogen) of the cubic boronnitride by way of the carbon. Since the carbon exhibits such a catalystfunction, neck growth is promoted to occur between the particles of theorganic cubic boron nitride powder, thereby increasing binding strengthbetween the cubic boron nitride grains in the cubic boron nitridesintered material. As a result, falling of the cubic boron nitridegrains are suppressed.

Therefore, according to the method of producing the cubic boron nitridesintered material according to one embodiment of the present disclosure,there can be produced a cubic boron nitride sintered material having along life with falling of the cubic boron nitride grains beingsuppressed, even though the cubic boron nitride sintered material is ahigh-cBN sintered material.

[2] In the method of producing the cubic boron nitride sinteredmaterial, the forming of the organic cubic boron nitride powder includesintroducing the cubic boron nitride source material powder and theorganic substance into supercritical water. This facilitates preparationof the organic cubic boron nitride powder in which the organic substanceis uniformly attached on the surfaces of the particles of the cubicboron nitride source material powder.

[3] In the method of producing the cubic boron nitride sinteredmaterial, the organic substance is an amine or a hydrocarbon compoundhaving a carbon number of more than or equal to 5. Accordingly, thefalling of the cubic boron nitride grains in the produced cubic boronnitride sintered material is dramatically reduced.

[4] In the method of producing the cubic boron nitride sinteredmaterial, the organic substance is hexylamine, hexanenitrile, paraffin,or hexane. Accordingly, the falling of the cubic boron nitride grains inthe produced cubic boron nitride sintered material is dramaticallyreduced.

[5] In the method of producing the cubic boron nitride sinteredmaterial, the forming of the organic cubic boron nitride powder includesattaching the organic substance onto the cubic boron nitride sourcematerial powder by plasma treatment. This facilitates preparation of theorganic cubic boron nitride powder in which the organic substance isuniformly attached on the surfaces of the particles of the cubic boronnitride source material powder.

[6] In the method of producing the cubic boron nitride sinteredmaterial, the organic substance is an amine or carbon fluoride. Thismakes it possible to prepare the organic cubic boron nitride powder inwhich the organic substance is uniformly attached on the surfaces of theparticles of the cubic boron nitride source material powder.

[7] A cubic boron nitride sintered material according to one embodimentof the present disclosure includes: more than or equal to 85 volume %and less than 100 volume % of cubic boron nitride grains; and aremainder of a binder, wherein the binder includes WC, Co and an Alcompound, and when a TEM-EDX is used to analyze an interface regionincluding an interface at which the cubic boron nitride grains areadjacent to each other, carbon exists on a whole or part of theinterface, and a width D of a region in which the carbon exists is morethan or equal to 0.1 nm and less than or equal to 10 nm.

In view of the content of the cubic boron nitride grains, it isunderstood that the cubic boron nitride sintered material is a “high-cBNsintered material” in which falling of the cubic boron nitride grains islikely to occur. However, the cubic boron nitride sintered material is acubic boron nitride sintered material produced by the above productionmethod. Hence, when the TEM-EDX is used to analyze the interface regionincluding the interface at which the cubic boron nitride grains areadjacent to each other, carbon exists on the interface, and the width Dof the region in which the carbon exists is more than or equal to 0.1 nmand less than or equal to 10 nm. In such a cubic boron nitride sinteredmaterial, binding strength between the cubic boron nitride grains isincreased as compared with the conventional cubic boron nitride sinteredmaterial. Therefore, the cubic boron nitride sintered material accordingto one embodiment of the present disclosure is a cubic boron nitridesintered material having a long life with falling of the cubic boronnitride grains being suppressed.

[8] In the cubic boron nitride sintered material, the width D is morethan or equal to 0.1 nm and less than or equal to 5 nm. In this case,the life of the cubic boron nitride sintered material can be longer.

[9] In the cubic boron nitride sintered material, a maximum value M of acontent of the carbon in the region in which the carbon exists is morethan or equal to 0.1 atom % and less than or equal to 5.0 atom %. Inthis case, the life of the cubic boron nitride sintered material can belonger.

[10] A cubic boron nitride sintered material according to anotherembodiment of the present disclosure includes: more than or equal to 85volume % and less than 100 volume % of cubic boron nitride grains; and aremainder of a binder, wherein the binder includes WC, Co and an Alcompound, when a TEM-EDX is used to analyze an interface regionincluding an interface at which the cubic boron nitride grains areadjacent to each other, carbon exists on a whole or part of theinterface, and a width D of a region in which the carbon exists is morethan or equal to 0.1 nm and less than or equal to 5 nm, and a maximumvalue M of a content of the carbon in the region in which the carbonexists is more than or equal to 0.1 atom % and less than or equal to 5.0atom %. With such a configuration, the cubic boron nitride sinteredmaterial according to one embodiment of the present disclosure is acubic boron nitride sintered material having a long life with falling ofthe cubic boron nitride grains being suppressed.

[11] A cubic boron nitride sintered material according to one embodimentof the present disclosure is a cutting tool including theabove-described cubic boron nitride sintered material. According to thecutting tool, a long life can be attained.

Details of Embodiments of the Present Disclosure

The following describes one embodiment (hereinafter, referred to as “thepresent embodiment”) of the present disclosure. The present embodimentis not limited thereto. It should be noted that in the presentspecification, the expression “A to Z” represents a range of lower toupper limits (i.e., more than or equal to A and less than or equal toZ). When no unit is indicated for A and a unit is indicated only for Z,the unit of A is the same as the unit of Z.

First Embodiment: Method of Producing Cubic Boron Nitride SinteredMaterial

A method of producing a cubic boron nitride sintered material accordingto the present embodiment will be described. The method of producing thecubic boron nitride sintered material according to the presentembodiment at least includes: forming an organic cubic boron nitridepowder by attaching an organic substance onto a cubic boron nitridesource material powder (forming step); preparing a powder mixtureincluding more than or equal to 85 volume % and less than 100 volume %of the organic cubic boron nitride powder and a remainder of a bindersource material powder by mixing the organic cubic boron nitride powderand the binder source material powder, the binder source material powderincluding WC, Co and Al (preparing step); and obtaining the cubic boronnitride sintered material by sintering the powder mixture (sinteringstep). Hereinafter, each step will be described in detail.

«Forming Step»

This step is a step of forming an organic cubic boron nitride powder byattaching an organic substance onto a cubic boron nitride sourcematerial powder.

The cubic boron nitride source material powder is a source materialpowder for the cubic boron nitride grains to be included in the cubicboron nitride sintered material. Examples of the method of attaching theorganic substance onto the cubic boron nitride source material powderinclude: a method employing supercritical water; a method of performingplasma treatment; and the like.

(Method Employing Supercritical Water)

The method employing supercritical water will be described. In themethod, a step of introducing the cubic boron nitride source materialpowder and the organic substance into supercritical water is performed.Accordingly, the organic cubic boron nitride powder can be formed. Itshould be noted that in the present specification, the supercriticalwater refers to water in a supercritical state or a subcritical state.

Examples of the method of introducing the cubic boron nitride sourcematerial powder and the organic substance into the supercritical waterinclude: a method of introducing the cubic boron nitride source materialpowder and the organic substance into the supercritical water in thisorder; a method of introducing the organic substance and the cubic boronnitride source material powder in this order; and a method ofintroducing the cubic boron nitride source material powder and theorganic substance simultaneously. According to each of these methods,the cubic boron nitride source material powder is brought into contactwith the supercritical water, thereby cleaning the surfaces of theparticles of the cubic boron nitride source material powder. Further,the cubic boron nitride source material powder including the particleshaving the cleaned surfaces and the organic substance are brought intocontact with each other, with the result that the organic substance isattached on the cleaned surfaces of the particles of the cubic boronnitride source material powder.

(Method of Performing Plasma Treatment)

The method of performing plasma treatment will be described. In themethod, a step of attaching the organic substance onto the cubic boronnitride source material powder by plasma treatment is performed.Specifically, in a plasma generating apparatus, the cubic boron nitridesource material powder is exposed to an atmosphere of first gasincluding carbon and is then exposed to an atmosphere of second gasincluding ammonia. As the first gas, CF₄, CH₄, C₂H₂ or the like can beused. As the second gas, a mixed gas of NH₃, N₂ and H₂, or the like canbe used.

According to the above method, by exposing the cubic boron nitridesource material powder to the atmosphere of first gas, the surfaces ofthe particles of the cubic boron nitride source material powder areetched, with the result that the cleaned surfaces are formed and thecarbon (first gas) is attached onto the cleaned surfaces. Then, thecubic boron nitride source material powder having the carbon attachedthereon is exposed to the atmosphere of second gas, with the result thatthe carbon is terminated by ammonia. As a result, the organic substanceincluding carbon and nitrogen is attached on the cleaned surfaces.

As described above, the organic cubic boron nitride powder can beefficiently formed by one of the method employing supercritical waterand the method of performing plasma treatment. In this step, it ispreferable to use the method employing supercritical water. This is dueto the following reason: the organic substance to be attached onto thecubic boron nitride source material powder can be readily made uniformand therefore the organic cubic boron nitride powder can be also readilymade uniform.

In this step, the average particle size of the cubic boron nitridesource material powder is not particularly limited. In order to form acubic boron nitride sintered material having high strength, high wearresistance, and high defect resistance, the average particle size of thecubic boron nitride source material powder is preferably 0.1 to 10 andis more preferably 0.5 to 5.0

When this step is performed using supercritical water, the organicsubstance to be used is preferably an amine or a hydrocarbon compoundhaving a carbon number of more than or equal to 5. Among them,hexylamine, hexanenitrile, paraffin and hexane are more preferable.Hexylamine is further preferable. The present inventors have confirmedthat when each of these organic substances is used, falling of the cubicboron nitride grains is dramatically reduced in the cubic boron nitridesintered material. When this step is performed using plasma treatment,examples of the organic substance to be attached include an amine,carbon fluoride, and the like.

A preferable amount of the organic substance to be attached onto thecubic boron nitride source material powder is changed depending on theparticle size of the cubic boron nitride source material powder. Forexample, when hexylamine is used as the organic substance, 50 to 2000ppm of hexylamine is preferably attached onto the cubic boron nitridesource material powder having an average particle size of 1 to 10 and100 to 5000 ppm of hexylamine is preferably attached onto the cubicboron nitride source material powder having an average particle size of0.1 to 1 In each of such cases, a desired cubic boron nitride sinteredmaterial tends to be efficiently produced. The amount of the organicsubstance attached on the organic cubic boron nitride powder can bemeasured by, for example, gas chromatography mass spectroscopy.

Here, in the present embodiment, in the organic cubic boron nitridepowder to be subjected to a below-described second step of the sinteringstep, carbon exists to such an extent that a sufficient catalystfunction can be exhibited. Further, the amount of the organic substanceattached on the cubic boron nitride source material powder tends to bedecreased in a subsequent step (for example, a below-describedpurification step, preparing step, or the like). Therefore, even whenthe amount of the organic substance attached on the cubic boron nitridesource material powder is different from the above-described amount, forexample, is an excessive amount, it is considered that a suitable amountof carbon can remain on the organic cubic boron nitride powder to besubjected to the second step by making appropriate adjustment duringeach treatment of the subsequent step. It should be noted that the cubicboron nitride sintered material produced using the organic cubic boronnitride powder in which the suitable amount of carbon remains is a cubicboron nitride sintered material according to a second embodimentdescribed later.

«Purification Step»

It is preferable to remove an impurity from the organic cubic boronnitride powder obtained by the above-described forming step, beforeusing the organic cubic boron nitride powder in the below-describedpreparing step. Examples of the impurity include an unreacted organicsubstance. By removing the unreacted organic substance, an unintendedreaction in the preparing step and/or sintering step can be suppressed.

For example, when the supercritical water is used, the organic cubicboron nitride powder is obtained as slurry. In this case, by performingcentrifugal separation onto the slurry, the organic cubic boron nitridepowder and the unreacted organic substance can be separated from eachother.

Further, heat treatment may be performed (for example, at more than orequal to 250° C., preferably more than or equal to 400° C., or morepreferably more than or equal to 850° C. in vacuum) onto the organiccubic boron nitride source material powder removed from thesupercritical water or the organic cubic boron nitride source materialpowder having been subjected to the centrifugal separation or the likeafter being removed from the supercritical water. Thus, an impurity suchas moisture on the surfaces of the particles of the organic cubic boronnitride powder can be removed.

Here, the present inventors have initially concerned that when the heattreatment is performed onto the organic cubic boron nitride powder, theorganic substance attached on the cubic boron nitride source materialpowder is entirely volatilized and/or ceases to exist. Surprisingly,however, as a result of observing the organic cubic boron nitride powderby Auger electron spectroscopy, it was confirmed that carbon uniformlyremains on the surfaces of the particles of the organic cubic boronnitride powder although the organic substance was decomposed due to theheat treatment. This carbon is considered to be originated from theorganic substance.

That is, it was confirmed that by performing the heat treatment onto theorganic cubic boron nitride powder, not only the impurity on thesurfaces of the particles of the organic cubic boron nitride powder isremoved but also the organic cubic boron nitride powder including theparticles having the modified surfaces on which the carbon was uniformlyattached was obtained. Although this mechanism is unknown, the presentinventors presume that since the cleaned surfaces formed by thetreatment with supercritical water, plasma, or the like have asignificantly high activity, the cleaned surfaces and the organicsubstance are bound to each other very strongly and this strong bindingis involved in the surface modification of the organic cubic boronnitride powder.

«Preparing Step»

This step is a step of preparing a powder mixture including more than orequal to 85 volume % and less than 100 volume % of the organic cubicboron nitride powder and a remainder of a binder source material powderby mixing the organic cubic boron nitride powder and the binder sourcematerial powder, the binder source material powder including WC, Co andAl. The organic cubic boron nitride powder is the organic cubic boronnitride powder obtained by the above-described forming step, and thebinder source material powder is a source material for the binder of thecubic boron nitride sintered material.

The binder source material powder can be prepared as follows. First, WCpowder, Co powder, and Al powder are prepared. Next, the powders aremixed at a predetermined ratio and are subjected to heat treatment (forexample, 1200° C.) in vacuum, thereby forming an intermetallic compound.The intermetallic compound is pulverized by a wet ball mill, a wet beadmill, or the like, thereby preparing the binder source material powderincluding WC, Co, and Al. It should be noted that the method of mixingthe powders is not particularly limited; however, in order toefficiently and uniformly mix the powders, ball mill mixing, bead millmixing, planetary mill mixing, jet mill mixing, or the like ispreferable. Each of the mixing methods may be performed in a wet manneror dry manner.

The organic cubic boron nitride powder and the prepared binder sourcematerial powder are preferably mixed by wet ball mill mixing employingethanol, acetone or the like as a solvent. After the mixing, the solventis removed by natural drying. Then, an impurity such as moisture on thesurfaces thereof is preferably removed by heat treatment (for example,at more than or equal to 850° C. in vacuum). Thus, on the surfaces ofthe particles of the organic cubic boron nitride powder, the organicsubstance is decomposed and the carbon originated from the organicsubstance can uniformly remain as described above, thereby obtaining theorganic cubic boron nitride powder including the particles having themodified surfaces. In this way, the powder mixture is prepared.

The binder source material powder may include other element(s) inaddition to WC, Co, and Al. Suitable examples of the other element(s)include Ni, Fe, Cr, Mn, Ti, V, Zr, Nb, Mo, Hf, Ta, Re, and the like.

«Sintering Step»

This step is a step of obtaining the cubic boron nitride sinteredmaterial by sintering the powder mixture. In this step, the powdermixture is sintered under a high-temperature and high-pressurecondition, thereby producing the cubic boron nitride sintered material.

Specifically, first, as a first step, the powder mixture is introducedinto a container and is vacuum-sealed. A vacuum sealing temperature ispreferably more than or equal to 850° C. This temperature is atemperature of more than the melting point of a sealing material, and isa sufficient temperature by which the organic substance attached on theorganic cubic boron nitride powder is decomposed and carbon originatedfrom the organic substance remains uniformly on the surfaces of theparticles of the organic cubic boron nitride powder.

Next, as a second step, the vacuum-sealed powder mixture is sinteredusing an ultra-high temperature and ultra-high pressure apparatus.Sintering conditions are not particularly limited, but are preferably5.5 to 8 GPa and more than or equal to 1500° C. and less than 2000° C.In particular, in view of balance between cost and sinteringperformance, 6 to 7 GPa and 1600 to 1900° C. are preferable.

In the case where the heat treatment (the heat treatment in thepurification step and/or the heat treatment in the preparing step) hasbeen performed before this step, the organic cubic boron nitride powderincluding the particles having the modified surfaces on which carbonremains uniformly is subjected to the first step. In the case where noheat treatment has been performed before this step, the organic cubicboron nitride powder including the particles having the modifiedsurfaces is prepared by the first step, i.e., the vacuum sealing.Therefore, carbon uniformly exists on the surfaces of the particles ofthe organic cubic boron nitride powder to be subjected to the secondstep. By performing the second step onto the powder mixture includingsuch an organic cubic boron nitride powder, the cubic boron nitridesintered material is produced.

«Function and Effect»

According to the above-described method of producing the cubic boronnitride sintered material according to the present embodiment, a cubicboron nitride sintered material having a long life can be produced. Thisis presumably due to the following reason: the carbon existing uniformlyon the surfaces of the particles of the organic cubic boron nitridepowder exhibits a catalyst function to promote occurrence of neck growthbetween the cubic boron nitride grains, with the result that the cubicboron nitride sintered material excellent in binding strength betweenthe cubic boron nitride grains is obtained.

Therefore, according to the method of producing the cubic boron nitridesintered material according to one embodiment of the present disclosure,there can be produced a cubic boron nitride sintered material having along life with falling of the cubic boron nitride grains beingsuppressed, even though the cubic boron nitride sintered material is ahigh-cBN sintered material. It should be noted that when carbon iscontained in the binder source material powder in the conventionalmethod of producing a high-cBN sintered material, the carbon does notexist uniformly on the surfaces of the cubic boron nitride grains andexists in an imbalanced manner between the cubic boron nitride grains.

Second Embodiment: Cubic Boron Nitride Sintered Material

A cubic boron nitride sintered material according to the presentembodiment will be described. The cubic boron nitride sintered materialaccording to the present embodiment is a cubic boron nitride sinteredmaterial produced by the above-described production method.

Specifically, the cubic boron nitride sintered material according to thepresent embodiment includes more than or equal to 85 volume % and lessthan 100 volume % of the cubic boron nitride grains and the remainder ofthe binder. That is, the cubic boron nitride sintered material accordingto the present embodiment is a so-called high-cBN sintered material. Itshould be noted that the cubic boron nitride sintered material mayinclude an inevitable impurity resulting from a source material usedherein, a production condition, or the like. In this case, it isunderstandable that the inevitable impurity is included in the binder.

The content ratio (volume %) of the cubic boron nitride grains in thecubic boron nitride sintered material is substantially the same as thecontent ratio (volume %) of the cubic boron nitride source materialpowder used in the powder mixture described later. This is because anamount of change in volume caused by the attachment of the organicsubstance or the like is very small with respect to the volume of thecubic boron nitride powder itself. Therefore, by controlling the contentratio of the cubic boron nitride source material powder used in thepowder mixture, the content (content ratio) of the cubic boron nitridegrains in the cubic boron nitride sintered material can be adjusted tofall in a desired range.

The content ratio (volume %) of the cubic boron nitride grains in thecubic boron nitride sintered material can also be confirmed byperforming quantitative analysis through inductively coupledhigh-frequency plasma spectrometry (ICP), or by performing structureobservation, element analysis, or the like onto the cubic boron nitridesintered material using an energy dispersive X-ray analyzer (EDX)accompanied with a scanning electron microscope (SEM) or an EDXaccompanied with a transmission electron microscope (TEM). In thepresent embodiment, unless otherwise specified, the content ratio of thecubic boron nitride grains in the cubic boron nitride sintered materialis determined by a below-described method using SEM.

For example, when the SEM is used, the content ratio (volume %) of thecubic boron nitride grains can be determined as follows. First, thecubic boron nitride sintered material is cut at an arbitrary position toform a sample including a cross section of the cubic boron nitridesintered material. For the formation of the cross section, a focused ionbeam device, a cross section polisher device, or the like can be used.Next, the cross section is observed by the SEM at a magnification of2000× to obtain a reflected electron image. In the reflected electronimage, a black region represents a region in which the cubic boronnitride grains exist and a gray or white region represents a region inwhich the binder exists.

Next, the reflected electron image is subjected to binarizationprocessing using image analysis software (for example, “WinROOF”provided by Mitani Corporation), and each of the area ratios iscalculated from the image having been through the binarizationprocessing. The calculated area ratio is regarded as volume %, therebyfinding the content ratio (volume %) of the cubic boron nitride grains.It should be noted that with this, the volume % of the binder can befound at the same time.

«Cubic Boron Nitride Grains»

The cubic boron nitride grains have high hardness, high strength, andhigh toughness, and serves as a base of the cubic boron nitride sinteredmaterial. D₅₀ (average grain size) of the cubic boron nitride grains isnot particularly limited, and may be, for example, 0.1 to 10.0 μm.Normally, as D₅₀ is smaller, the hardness of the cubic boron nitridesintered material tends to be higher. Moreover, as variation in thegrain sizes is smaller, the characteristics of the cubic boron nitridesintered material tend to be more uniform. D₅₀ of the cubic boronnitride grains is preferably, for example, 0.5 to 4.0 μm.

D₅₀ of the cubic boron nitride grains is determined as follows. First, asample including a cross section of the cubic boron nitride sinteredmaterial is formed in a manner similar to that in the above-describedmethod of finding the content of the cubic boron nitride grains, and areflected electron image is obtained. Next, the equivalent circlediameter of each black region in the reflected electron image iscalculated using image analysis software. It is preferable to calculatethe equivalent circle diameters of 100 or more cubic boron nitridegrains by performing observation in five or more visual fields.

Next, the equivalent circle diameters are arranged in an ascending orderfrom the minimum value to the maximum value to find a cumulativedistribution. D₅₀ represents a grain size corresponding to a cumulativearea of 50% in the cumulative distribution. It should be noted that theequivalent circle diameter refers to the diameter of a circle having thesame area as the area of the measured cubic boron nitride grain.

«Binder»

The binder serves to sinter cubic boron nitride particles at industriallevels of pressure and temperature. Each of the cubic boron nitrideparticles is a difficult-to-be-sintered material. Further, since thebinder has lower reactivity with respect to iron than that of cubicboron nitride, the binder provides the cubic boron nitride sinteredmaterial with a function of suppressing chemical wear and thermal wearin cutting of high-hardness hardened steel. When the cubic boron nitridesintered material includes the binder, wear resistance inhigh-efficiency processing of high-hardness hardened steel is improved.

In the cubic boron nitride sintered material of the present embodiment,the binder includes WC, Co, and an Al compound. Here, the “Al compound”refers to a compound including Al as a constituent element. Examples ofthe Al compound include CoAl, Al₂O₃, AlN, AlB₂, composite compoundsthereof, and the like. Due to the following reasons, the binderincluding WC, Co and the Al compound is considered to be particularlyeffective in attaining a long life of the cubic boron nitride sinteredmaterial according to the present embodiment.

First, since each of Co and Al has a catalyst function, neck growthbetween the cubic boron nitride grains can be promoted in the sinteringstep. Second, WC is presumed to be effective in providing the binderwith a thermal expansion coefficient close to the thermal expansioncoefficient of the cubic boron nitride grains. It should be noted thatthe catalyst function means that B (boron) and/or N (nitrogen) of thecubic boron nitride grains is diffused or precipitated by way of Co orAl.

The composition of the binder included in the cubic boron nitridesintered material can be specified by combining XRD (X-ray diffractionmeasurement) and ICP. Specifically, first, a specimen having a thicknessof about 0.45 to 0.50 mm is cut from the cubic boron nitride sinteredmaterial, and XRD analysis is performed onto the specimen to determine acompound, a metal, or the like based on an X-ray diffraction peak. Next,the specimen is immersed in hydrofluoric-nitric acid (acid mixture withconcentrated nitric acid (60%):distilled water:concentrated hydrofluoricacid (47%)=2:2:1 at a volume ratio) within a sealed container, therebyobtaining an acid-treated solution having the binder dissolved therein.The acid-treated solution is subjected to ICP analysis to performquantitative analysis for each metal element. The composition of thebinder is determined by analyzing the results of XRD and ICP analysis.

The binder in the present embodiment may include other binder(s) inaddition to WC, Co, and the Al compound. Suitable examples of theelement(s) of the other binder(s) include Ni, Fe, Cr, Mn, Ti, V, Zr, Nb,Mo, Hf, Ta, Re, and the like.

«Analysis with TEM-EDX»

A feature of the cubic boron nitride sintered material according to thepresent embodiment lies in that the following conditions (1) and (2) aresatisfied when the interface region including the interface at which thecubic boron nitride grains are adjacent to each other is analyzed usingTEM-EDX:

(1) Carbon exists on the interface; and

(2) Width D of the region in which the carbon exists is 0.1 to 10 nm.

The analysis using TEM-EDX is performed as follows. First, a sample isobtained from the cubic boron nitride sintered material, and an argonion slicer is used to slice the sample to form a cut piece having athickness of 30 to 100 nm. Then, the cut piece is observed using a TEM(transmission electron microscope) at a magnification of 50000×, therebyobtaining a first image. Examples of the transmission electronmicroscope used on this occasion include “JEM-2100F/Cs” (trademark)provided by JEOL. In the first image, one interface at which the cubicboron nitride grains are adjacent to each other is arbitrarily selected.Next, the selected interface is positioned to pass through the vicinityof the center of the image, and observation is performed at anobservation magnification changed to 2000000×, thereby obtaining asecond image. In the obtained second image (100 nm×100 nm), theinterface exists to extend from one end of the image to the other oneend of the image opposite to the foregoing one end, via the vicinity ofthe center of the image.

Next, element mapping analysis is performed onto the second image usingEDX so as to analyze the distribution of carbon in the second image,i.e., in the interface region including the interface. Examples of theenergy dispersive X-ray analyzer used on this occasion include “EDAX”(trademark) provided by AMETEK. When a region having a highconcentration of carbon is observed on the interface (to coincide withthe shape of the interface), the cubic boron nitride sintered materialsatisfies the above-described condition (1).

In the second image of the cubic boron nitride sintered materialsatisfying the condition (1), an extending direction in which theinterface extends (extending direction in which the region having a highconcentration of carbon extends) is confirmed, and then element lineanalysis is performed in a direction substantially perpendicular to theextension direction. A beam diameter on that occasion is less than orequal to 0.3 nm, and a scanning interval is 0.1 to 0.7 nm. Width D ofthe region in which carbon exists is calculated in accordance with theresult of the element line analysis. When width D is 0.1 to 10 nm, thecubic boron nitride sintered material satisfies the above-describedcondition (2).

The above-described analyses are repeated in first images correspondingto six visual fields. When it is confirmed that the conditions (1) and(2) are satisfied in one or more visual fields, the cubic boron nitridesintered material can be regarded as the cubic boron nitride sinteredmaterial according to the present embodiment. On this occasion, thecondition (1) can be recognized also as “carbon exists on a whole orpart of the interface”.

The above analyses will be described more in detail with reference toFIGS. 1 to 5 in order to facilitate understanding.

FIG. 1 shows an exemplary second image. Referring to FIG. 1, a blackregion corresponds to a region (BN region) including B and N as mainconstituent elements, and a white region or a gray region corresponds toa region (SF region) recognized as the interface in the first image. Asshown in FIG. 1, the SF region in the second image corresponds to the“interface at which the cubic boron nitride grains are adjacent to eachother”, and the whole of the second image corresponds to the “interfaceregion including the interface”.

Here, when the width of the SF region (in a substantiallyupward/downward direction in FIG. 1) is more than 10 nm in the secondimage, one different interface is reselected in the first image. This isdue to the following reason: when the width of the SF region is morethan 10 nm, it is difficult to say that the SF region corresponds to the“interface at which the cubic boron nitride grains are adjacent to eachother”.

FIGS. 2 to 4 show results of performing the element mapping analysisonto the second image shown in FIG. 1 using EDX. FIGS. 2 to 4 showdistribution states of boron, nitrogen, and carbon, respectively. In theelement mapping analysis, a position at which an element exists indicatea light color. Thus, in each of FIGS. 2 to 4, a region indicating a darkcolor is a region in which a corresponding element does not exist (or avery small amount of the corresponding element exists). As a region hasa lighter color, a larger amount of the corresponding element exists inthe region.

Referring to FIGS. 2 and 3, the distribution amounts of boron andnitrogen in the SF region are decreased as compared with thedistribution amounts of B and N in the BN region. On the other hand, inview of FIG. 4, it is understandable that carbon exists in the SFregion. In FIG. 4, the region in which carbon exists (hereinafter, alsoreferred to as “carbon-containing region”) extends in theleftward/rightward direction in the figure and substantially coincideswith the SF region.

A white solid line indicated in the image shown in FIG. 4 represents aresult of performing the element line analysis in the direction(upward/downward direction in FIG. 4) substantially perpendicular to theextending direction (leftward/rightward direction in FIG. 4) of thecarbon-containing region. FIG. 5 shows this in the form of a graph. FIG.5 shows a result in a solid line with the horizontal axis representing adistance (nm) in which the line analysis is performed and with thevertical axis representing a value of the carbon content (atom %) at aspot as calculated in accordance with the result of the line analysis.Moreover, for the sake of reference, FIG. 5 shows a result in a dottedline with the horizontal axis representing the distance (nm) in whichthe line analysis is performed and with the vertical axis representingan intensity (a.u.) of an HAADF (High-Angle Annular Dark Field) image.

Referring to FIG. 5, in a region of the HAADF image having a highintensity, i.e., in the interface region, a peak of the content ratio(atom %) of carbon is observed. The portion at which the peak isobserved is the “region in which carbon exists”, and width d of the peakis “width D of the region in which carbon exists”.

«Function and Effect»

According to the cubic boron nitride sintered material according to thepresent embodiment, a long life can be attained. This is due to thefollowing reason: since the cubic boron nitride sintered materialaccording to the present embodiment is a cubic boron nitride sinteredmaterial produced by the production method according to the firstembodiment, carbon exists uniformly on the interface between the cubicboron nitride grains, thereby increasing binding strength between thecubic boron nitride grains. According to various studies, it has beenconfirmed that the above-described conditions (1) and (2) are preferablysatisfied in three or more of the six visual fields observed in theabove-described method.

On the other hand, when width D is more than 10 nm, a cubic boronnitride sintered material having a long life cannot be obtained. This ispresumably due to the following reason. That is, when an amount ofcarbon remaining on each of the surfaces of the cubic boron nitridegrains is too large, width D becomes more than 10 nm. In this case, anexcess of free carbon exists in the cubic boron nitride grain, thusresulting in decreased binding strength between the cubic boron nitridegrains. On the other hand, also when width D is less than 0.1 nm, acubic boron nitride sintered material having a long life cannot beobtained. This is presumably due to the following reason: the amount ofcarbon existing on each of the surfaces of the cubic boron nitridegrains is too small and is therefore not sufficient to improve thebinding strength between the cubic boron nitride grains. Further, widthD of the cubic boron nitride sintered material according to the presentembodiment is preferably 0.1 to 5 nm. In this case, the life of thecubic boron nitride sintered material can be longer.

It should be noted that carbon may exist between the cubic boron nitridegrains also in the conventional high-cBN sintered material. However,this carbon is originated from the binder, and therefore exists in animbalanced manner between the cubic boron nitride grains and does notexist uniformly between the cubic boron nitride grains. The width of theregion in which carbon exists in the imbalanced manner is large to beabout 0.1 to 2.0 μm and does not satisfy the above-described condition(2).

In the cubic boron nitride sintered material according to the presentembodiment, maximum value M of the content of the carbon in the regionin which the carbon exists (the carbon-containing region of the secondimage) is preferably 0.1 to 5.0 atom %. In this case, the life of thecubic boron nitride sintered material can be longer. Maximum value M ofthe content of the carbon is the maximum value among the content ratios(atom %) of carbon at respective spots as calculated in accordance withthe result of the line analysis. For example, in FIG. 5, maximum value Mof the content of the carbon in the carbon-containing region is about1.4 atom %.

Meanwhile, when maximum value M is less than 0.1 atom %, theabove-described effect may not be suitably exhibited. On the other hand,when maximum value M is more than 5.0 atom %, an excess of free carbonexists, thus presumably resulting in decreased binding strength betweenthe grains, conversely.

Particularly when width D is 0.1 to 5 nm and maximum value M is 0.1 to5.0 atom %, the cubic boron nitride sintered material according to thepresent embodiment has a significantly long life.

Third Embodiment: Cutting Tool

A cutting tool according to the present embodiment includes theabove-described cubic boron nitride sintered material. In one aspect ofthe present embodiment, the cutting tool includes the cubic boronnitride sintered material as a substrate. The cutting tool according tothe present embodiment may have a coating film on a surface of the cubicboron nitride sintered material serving as a substrate.

The shape and application of the cutting tool according to the presentembodiment are not particularly limited. Examples of the cutting toolaccording to the present embodiment include a drill, an end mill, anindexable cutting insert for drill, an indexable cutting insert for endmill, an indexable cutting insert for milling, an indexable cuttinginsert for turning, a metal saw, a gear cutting tool, a reamer, a tap,an insert for crankshaft pin milling, and the like.

Further, the cutting tool according to the present embodiment is notlimited to a cutting tool entirely composed of the cubic boron nitridesintered material, and includes a cutting tool having a portion(particularly, a cutting edge portion or the like) composed of the cubicboron nitride sintered material. For example, the cutting tool accordingto the present embodiment also includes a cutting tool in which a basebody (supporting body) composed of a cemented carbide or the like has acutting edge portion composed of the cubic boron nitride sinteredmaterial. In this case, the cutting edge portion is literally regardedas a cutting tool. In other words, even when the cubic boron nitridesintered material constitutes only a portion of the cutting tool, thecubic boron nitride sintered material is referred to as a cutting tool.

According to the cutting tool according to the present embodiment, thecutting tool includes the above-described cubic boron nitride sinteredmaterial, and therefore has a long life.

The above description includes features described below.

(Clause 1)

A method of producing a cubic boron nitride sintered material, themethod comprising:

forming an organic cBN powder by attaching an organic substance onto acBN source material powder;

preparing a powder mixture including more than or equal to 85 volume %and less than 100 volume % of the organic cBN powder and a remainder ofa binder source material powder by mixing the organic cBN powder and thebinder source material powder, the binder source material powderincluding WC, Co and Al; and

obtaining the cBN sintered material by sintering the powder mixture.

(Clause 2)

The method of producing the cubic boron nitride sintered materialaccording to clause 1, wherein the forming of the organic cBN powderincludes introducing the cBN source material powder and the organicsubstance into supercritical water.

(Clause 3)

The method of producing the cubic boron nitride sintered materialaccording to clause 1, wherein the forming of the organic cBN powderincludes attaching the organic substance onto the cBN source materialpowder by plasma treatment.

(Clause 4)

A cubic boron nitride sintered material comprising: more than or equalto 85 volume % and less than 100 volume % of cBN grains; and a remainderof a binder, wherein

the binder includes WC, Co and an Al compound, and

when a TEM-EDX is used to analyze an interface region including aninterface at which the cBN grains are adjacent to each other,

-   -   carbon exists on the interface, and    -   a width D of a region in which the carbon exists is more than or        equal to 0.1 nm and less than or equal to 10 nm.

(Clause 5)

The cubic boron nitride sintered material according to clause 4, whereinthe width D is more than or equal to 0.1 nm and less than or equal to 5nm.

(Clause 6)

The cubic boron nitride sintered material according to clause 4 or 5,wherein a maximum value M of a content of the carbon in the region inwhich the carbon exists is more than or equal to 0.1 atom % and lessthan or equal to 5.0 atom %.

(Clause 7)

A cutting tool comprising the cubic boron nitride sintered materialrecited in any one of clauses 4 to 6.

EXAMPLES

Hereinafter, the present invention will be described more in detail withreference to Examples, but the present invention is not limited thereto.

Experiment Example 1

First, an organic cubic boron nitride powder was formed. Specifically,first, supercritical water was formed using a supercritical watersynthesis apparatus (“Momicho mini” provided by ITEC) under thefollowing conditions.

Pressure: 35 MPa

Temperature: 375° C.

Flow Rate: 2 ml/min.

Next, a hexylamine source solution was continuously introduced into theapparatus to attain a hexylamine concentration of 10.0 weight % in thesupercritical water. Further, a cubic boron nitride source materialpowder having an average particle size of 2 μm was continuouslyintroduced into the apparatus to attain an amount of the cubic boronnitride source material powder of 10 weight % in the supercriticalwater. In this way, the cubic boron nitride source material powder andthe organic substance were introduced into the supercritical water.

After the above-described supercritical water treatment was continuedfor 100 minutes, the temperature and pressure in the apparatus wasreturned to normal temperature and pressure to end the preparing step,and a whole of the obtained slurry was collected. The slurry wascentrifuged (at 10000 rpm for 5 minutes) to separate an excess ofhexylamine not attached on the cubic boron nitride source materialpowder. The concentrated slurry after the separation was dried (at −90°C. for 12 hours) to collect about 20 g of powder having been through thesupercritical water treatment.

In this way, the organic cubic boron nitride powder was formed. Theformed organic cubic boron nitride powder was subjected to gaschromatography mass spectroscopy, thus confirming that 895 ppm ofhexylamine existed (attached) with respect to the cubic boron nitridepowder.

Next, a binder source material powder serving as a source material ofthe binder was prepared. Specifically, WC powder, Co powder, and Alpowder were prepared, and they were blended at a ratio ofWC:Co:Al=50:40:10 in weight %. It should be noted that the averageparticle size of each powder was 2 The mixture was made uniform by heattreatment (at 1200° C. for 30 minutes in vacuum), and was thenpulverized into fine particles by a carbide ball mill. In this way, abinder source material powder having an average particle size of 1 μmwas obtained.

The organic cubic boron nitride powder and the obtained binder sourcematerial powder were blended at the following ratio: the organic cubicboron nitride powder:the binder source material powder=85:15 in volume%. Then, they were mixed uniformly by the wet ball mill method usingethanol. Thereafter, the mixed powder was subjected to heat treatment at900° C. in vacuum. The organic cubic boron nitride powder having beenthrough the heat treatment was analyzed by Auger electron spectroscopy,thus confirming that carbon remained on the surfaces of the particles ofthe organic cubic boron nitride powder. In this way, a powder mixturewas formed.

Next, the obtained powder mixture was sintered to form a cubic boronnitride sintered material. Specifically, the powder mixture wasintroduced into a container composed of Ta with the powder mixture beingin contact with a WC-6% Co cemented carbide disc and a Co foil, and wasvacuum-sealed. This powder mixture was sintered at 7.0 GPa and 1700° C.for 15 minutes using a belt-type ultra-high pressure and ultra-hightemperature generating apparatus. In this way, the cubic boron nitridesintered material was formed.

Experiment Example 2

The concentration of hexylamine to be introduced was set to 8.0 weight%. The organic cubic boron nitride powder and the binder source materialpowder were blended at the following ratio: the organic cubic boronnitride powder:the binder source material powder=95:5 in volume %. Then,they were uniformly mixed by the wet ball mill method using ethanol.Thereafter, heat treatment was performed onto the mixed powder at 250°C. in vacuum. Except for the above, a cubic boron nitride sinteredmaterial was formed in the same manner as in Experiment Example 1. Theorganic cubic boron nitride powder was subjected to gas chromatographymass spectroscopy, thus confirming that 821 ppm of hexylamine existedwith respect to cubic boron nitride.

Experiment Example 3

The concentration of hexylamine to be introduced was set to 6.0 weight%. The organic cubic boron nitride powder and the binder source materialpowder were blended at the following ratio: the organic cubic boronnitride powder:the binder source material powder=92:8 in volume %. Then,they were uniformly mixed by the wet ball mill method using ethanol.Thereafter, heat treatment was performed onto the mixed powder at 400°C. in vacuum. Except for the above, a cubic boron nitride sinteredmaterial was formed in the same manner as in Experiment Example 1. Theorganic cubic boron nitride powder was subjected to gas chromatographymass spectroscopy, thus confirming that 543 ppm of hexylamine existedwith respect to cubic boron nitride.

Experiment Example 4

The concentration of hexylamine to be introduced was set to 4.0 weight%, and the organic cubic boron nitride powder and the binder sourcematerial powder were blended at the following ratio: the organic cubicboron nitride powder:the binder source material powder=92:8 in volume %.Except for the above, a cubic boron nitride sintered material was formedin the same manner as in Experiment Example 1. The organic cubic boronnitride powder was subjected to gas chromatography mass spectroscopy,thus confirming that 302 ppm of hexylamine existed with respect to cubicboron nitride.

Experiment Example 5

The concentration of hexylamine to be introduced was set to 13.0 weight%, and the organic cubic boron nitride powder and the binder sourcematerial powder were blended at the following ratio: the organic cubicboron nitride powder:the binder source material powder=92:8 in volume %.Except for the above, a cubic boron nitride sintered material was formedin the same manner as in Experiment Example 1. The organic cubic boronnitride powder was subjected to gas chromatography mass spectroscopy,thus confirming that 1343 ppm of hexylamine existed with respect tocubic boron nitride.

Experiment Example 6

Instead of the method employing the supercritical water, the organiccubic boron nitride powder was formed by plasma treatment. Specifically,the surfaces of the particles of the cubic boron nitride source materialpowder were etched in a CF₄ atmosphere using a plasma modificationapparatus (low-pressure plasma apparatus FEMTO provided by Dienner), andthen the atmosphere in the apparatus was changed to an NH₃ atmospherefor the purpose of treatment of the cubic boron nitride source materialpowder having been etched. Except for the above, the cubic boron nitridesintered material was produced in the same manner as in ExperimentExample 1.

Experiment Example 7

A cubic boron nitride sintered material was produced in the same manneras in Experiment Example 2 except that the plasma treatment was usedinstead of the method employing supercritical water.

Experiment Example 8

A cubic boron nitride sintered material was produced in the same manneras in Experiment Example 3 except that the plasma treatment was usedinstead of the method employing supercritical water.

Experiment Example 9

A cubic boron nitride sintered material was produced in the same manneras in Experiment Example 4 except that the plasma treatment was usedinstead of the method employing supercritical water.

Experiment Example 10

A cubic boron nitride sintered material was produced in the same manneras in Experiment Example 5 except that the plasma treatment was usedinstead of the method employing supercritical water.

Experiment Example 21

A cubic boron nitride sintered material was produced in the same manneras in Experiment Example 3 except that the powder mixture was preparedusing a cubic boron nitride source material powder without the treatmentemploying supercritical water.

Experiment Example 22

A cubic boron nitride sintered material was formed in the same manner asin Experiment Example 4 except that the organic cubic boron nitridepowder and the binder source material powder were blended at thefollowing ratio: the organic cubic boron nitride powder:the bindersource material powder=65:35 in volume %.

Experiment Example 23

A cubic boron nitride sintered material was formed in the same manner asin Experiment Example 1 except that the treatment employingsupercritical water was not performed and only the cubic boron nitridesource material powder was used with no binder source material powderbeing blended.

In the manner described above, the cubic boron nitride sinteredmaterials of Experiment Examples 1 to 10 and Experiment Examples 21 to23 were formed. Here, Experiment Examples 1 to 10 correspond to examplesof the present disclosure.

Experiment Examples 21 to 23 correspond to comparative examples.

<Evaluation on Characteristics>

«Width D and Maximum Value M»

Each of the formed cubic boron nitride sintered materials was cut at anappropriate position, and then its exposed surface was polished to forma smooth surface. Thereafter, an argon ion slicer was used to slice thecubic boron nitride sintered material to form a cut piece having athickness of 50 nm. Next, element mapping analysis and element lineanalysis by EDX were performed onto a second image (100 nm×100 nm) inaccordance with the methods described above. On this occasion,“JEM-2100F/Cs” (trademark) provided by JEOL was used as a transmissionelectron microscope. As the energy dispersive X-ray analyzer, EDAX(trademark) provided by AMETEK was used. A beam diameter in the EDX was0.2 nm and a scanning interval was 0.6 nm. Analysis Station provided byJEOL was used as software for the element mapping analysis and elementline analysis by EDX. Results thereof are shown in Table 1.

It should be noted that each value shown in Table 1 represents anaverage value in the visual fields in each of which the above-describedconditions (1) and (2) are satisfied. In each of the cut pieces ofExperiment Examples 1 and 4 to 10, the above-described conditions (1)and (2) were satisfied in all of six arbitrarily extracted interfaceregions. In Experiment Example 2, the above-described conditions (1) and(2) were satisfied in one visual field of six arbitrarily extractedinterface regions. In Experiment Example 3, the above-describedconditions (1) and (2) were satisfied in three visual fields of sixarbitrarily extracted interface regions.

«Composition of Binder»

A specimen having a length of 6 mm, a width of 3 mm, and a thickness of0.45 to 0.50 mm was cut from each of the formed cubic boron nitridesintered materials, and XRD analysis was performed onto the specimen.Next, each specimen was immersed in hydrofluoric-nitric acid (acidmixture with concentrated nitric acid (60%):distilled water:concentratedhydrofluoric acid (47%)=2:2:1 at a volume ratio) at 140° C. for 48 hourswithin a sealed container, thereby obtaining an acid-treated solutionhaving the binder dissolved therein. The acid-treated solution wassubjected to ICP analysis. The composition of the binder was specifiedin accordance with the results of XRD analysis and ICP analysis.

«Bending Strength»

The bending strength (GPa) of each specimen having been through the acidtreatment was measured using a three-point bending tester at a strokespeed of 0.5 mm/min with a span of 4 mm. Results thereof are shown inTable 1.

«Cutting Test»

A cutting tool (substrate shape: CNGA120408; cutting edge treatment:T01215) was formed using each of the formed cubic boron nitride sinteredmaterials. A cutting test was performed using this cutting tool underthe following cutting conditions.

Cutting speed: 150 m/min.

Feeding speed: 0.05 mm/rev.

Depth of cut: 0.1 mm

Coolant: DRY

Cutting method: intermittent cutting

Lathe: LB400 (provided by OKUMA Corporation)

Workpiece: sintered component (hardened sintered alloy D-40 provided bySumitomo Electric Industries with a hardened cutting portion having ahardness of 40 HRC).

The cutting edge was observed per cutting distance of 0.4 km so as tomeasure an amount of falling from the cutting edge. The amount offalling from the cutting edge was defined as a reduced width from theposition of the ridgeline of the cutting edge before the cutting. Thereduced width results from wear. In the case of occurrence of chipping,the size of chipping was defined as the amount of falling. A cuttingdistance at a point of time at which the amount of falling from thecutting edge became more than or equal to 0.05 mm was measured. Itshould be noted that this cutting distance was defined as an index ofthe life of the cutting tool. Results thereof are shown in Table 1.

TABLE 1 Width Maximum Bending Cutting cBN D Value M Strength Distance(Volume %) (nm) (Atom %) (GPa) (km) Experiment 85 5.0 7.2 0.51 1.53Example 1 Experiment 95 0.1 0.1 0.66 1.76 Example 2 Experiment 92 2.14.0 0.85 1.92 Example 3 Experiment 92 5.0 5.0 0.82 1.82 Example 4Experiment 92 10.0 5.0 0.54 1.52 Example 5 Experiment 85 5.0 7.5 0.461.41 Example 6 Experiment 95 2.3 4.3 0.75 1.62 Example 7 Experiment 922.2 4.7 0.52 1.75 Example 8 Experiment 92 5.0 4.5 0.74 1.68 Example 9Experiment 92 8.2 4.8 0.49 1.48 Example 10 Experiment 92 — — 0.43 0.60Example 21 Experiment 65 — — Collapsed 0.48 Example 22 Experiment 100 —— Collapsed 0.32 Example 23

Table 1 also shows the volume % of cubic boron nitride in the cubicboron nitride sintered material. Further, regarding the composition ofthe binder, it was confirmed that at least WC, Co, and Al compoundexisted in each of Experiment Examples 1 to 10 and Experiment Examples21 to 22. Since no distinct peak was detected in XRD with regard to theAl compound, it was presumed that the Al compound was a compositecompound composed of a plurality of Al compounds.

Referring to Table 1, in each of Experiment Examples 1 and 4 to 10, theabove-described conditions (1) and (2) were satisfied in all the sixvisual fields of six arbitrarily extracted interfaces. In ExperimentExample 2, the above-described conditions (1) and (2) were satisfied inone visual field of six arbitrarily extracted interface regions. InExperiment Example 3, the above-described conditions (1) and (2) weresatisfied in three visual fields of six arbitrarily extracted interfaceregions. Therefore, the average value thereof is shown in each of thecolumns of Experiment Examples 1 and 3 to 10. On the other hand, in eachof Experiment Examples 21 to 23, no carbon-containing region wasobserved in all the six visual fields of six arbitrarily extractedinterfaces. Therefore, the above-described conditions (1) and (2) werenot satisfied therein. Hence, “-” is indicated in each of the columns ofExperiment Examples 21 to 23.

Referring to Table 1, it was confirmed that the cubic boron nitridesintered materials of Experiment Examples 1 to 10 have higher bendingstrengths than those of the cubic boron nitride sintered materials ofExperiment Examples 21 to 23. It should be noted that “Collapsed” ineach of Experiment Examples 22 and 23 means that the cubic boron nitridesintered material having been through the acid treatment wasspontaneously broken. Regarding each of the cubic boron nitride sinteredmaterials of Experiment Examples 1 to 10, it is understood that thecubic boron nitride grains are bound together strongly because thebending strength of the specimen having been through the acid treatmentis high. This result also supports the hypothesis that the existence ofcarbon promotes neck growth between the cubic boron nitride grains.

Further, in each of Experiment Examples 1 to 10, the cutting distance issignificantly longer than that in each of Experiment Examples 21 to 23.In view of this, it was confirmed that the lives of the cubic boronnitride sintered materials according to Experiment Examples 1 to 10 weresignificantly long. Among them, in Experiment Examples 2 to 4 and 7 to9, the cutting distances are significantly long. Hence, it was confirmedthat when width D is 0.1 to 5.0 nm and maximum value M is 0.1 to 5.0atom %, the life can be more significantly long.

The embodiments disclosed herein are illustrative and non-restrictive inany respect. The scope of the present invention is defined by the termsof the claims, rather than the embodiments described above, and isintended to include any modifications within the scope and meaningequivalent to the terms of the claims.

The invention claimed is:
 1. A cubic boron nitride sintered materialcomprising: more than or equal to 85 volume % and less than 100 volume %of cubic boron nitride grains; and a remainder of a binder, wherein thebinder includes WC, Co and an Al compound, and when a TEM-EDX is used toanalyze an interface region including an interface at which the cubicboron nitride grains are adjacent to each other, carbon exists on awhole or part of the interface, and a width D of a region in which thecarbon exists is more than or equal to 0.1 nm and less than or equal to10 nm.
 2. The cubic boron nitride sintered material according to claim1, wherein the width D is more than or equal to 0.1 nm and less than orequal to 5 nm.
 3. The cubic boron nitride sintered material according toclaim 1, wherein a maximum value M of a content of the carbon in theregion in which the carbon exists is more than or equal to 0.1 atom %and less than or equal to 5.0 atom %.
 4. A cubic boron nitride sinteredmaterial comprising: more than or equal to 85 volume % and less than 100volume % of cubic boron nitride grains; and a remainder of a binder,wherein the binder includes WC, Co and an Al compound, when a TEM-EDX isused to analyze an interface region including an interface at which thecubic boron nitride grains are adjacent to each other, carbon exists ona whole or part of the interface, and a width D of a region in which thecarbon exists is more than or equal to 0.1 nm and less than or equal to5 nm, and a maximum value M of a content of the carbon in the region inwhich the carbon exists is more than or equal to 0.1 atom % and lessthan or equal to 5.0 atom %.
 5. A cutting tool comprising the cubicboron nitride sintered material recited in claim 1.